In the manufacture of continuous glass filaments, glass is melted in a glass furnace and flows through a forehearth to one or more bushings in a filament forming apparatus. Each bushing has several nozzles or tips through which streams of molten glass flow. The quenched glass streams are mechanically pulled from the nozzles by a winding apparatus to form continuous glass filaments.
Conventional bushings may be either a remelt bushing or a direct-melt bushing. A remelt bushing melts cold glass in the form of marbles or other shapes in its upper section and then conditions the glass and passes it through the bushing nozzles from which the molten glass is attenuated. A direct-melt bushing is supplied with liquid glass at the desired temperature from a continuous supply flowing above the bushing in a channel called a forehearth. The direct-melt bushing only needs to condition the molten glass to a uniform temperature before it is attenuated.
A conventional filament forming apparatus 5 with a bushing is shown in FIG. 1 and is disclosed in U.S. Pat. No. 3,920,430 to Carey (Carey), the disclosure of which is expressly incorporated herein by reference. Filaments 20 are drawn from a plurality of nozzles 12 depending from a bottom plate 14 of the bushing 10 and are gathered into a strand 22 by a roller 42. Size is applied to coat the filaments by a size applicator 40. A reciprocating device 32 guides strand 22, which is wound around a rotating collet 34 in a winding apparatus 30 to build a cylindrical package 24.
The electrically heated bushing 10 is located below and in communication with a forehearth 50 which receives refined, heat-softened or molten glass from a melting furnace 52. The bushing 10 is mounted in communication with an opening in the bottom of the forehearth 50.
Conventional bushings include side walls, end walls, and a bottom plate defining a bushing body therebetween. The bottom plate may include more than 4,000 nozzles, preferably all at or close to a uniform temperature. The bottom plate may be referred to as a nozzle plate or tip plate as well.
Bushings condition the molten glass to a uniform temperature so the filaments are attenuated with uniform diameters. The temperature of the molten glass must be high enough to maintain the glass in a liquid state. Accordingly, bushings are subjected to high temperatures over their operating life.
As they lose heat to the ambient, the filaments are attenuated from the bushing nozzles by a winding apparatus that winds one or more packages simultaneously. In order to supply a sufficient amount of filaments to a winding apparatus, bushings have increased in size.
Larger bushings encounter several problems due to their size. It is difficult to maintain the nozzle plate of a large bushing at a uniform temperature and achieve uniform diameter filaments. Also, it is difficult to minimize and control the distortion of the nozzle plate due to high operating temperatures and the weight of the molten glass in the bushing above the plate. Larger bottom plates are subjected to a greater overall load and tend to sag or creep sooner than smaller bottom plates. Hotter operating temperatures in the current state of the art processes also accelerate the high temperature creep of the tip plate alloys.
These problems result in creep and distortion of the bottom plate, which limit the useful life of a bushing. Creep is the deformation of the nozzle plate under a load and is a function of temperature and the stress on the plate. Distortion results when an insufficient allowance is made for the thermal expansion of the plate when the temperature in the bushing increases.
Several attempts have been made to solve these problems. One proposed solution involves the particular material of the bushing. Platinum materials may be used because they are resistant to oxidation and to corrosion by the glass and as a result, do not contaminate the glass. Platinum alloys are resistant to creep at elevated temperatures as well. However, pure platinum is soft and quickly distorts at high temperatures. While the addition to the platinum of an alloying material such as rhodium has proved beneficial, the particular material of the bushing alone is insufficient to eliminate the creep and distortion in the bushing bottom plate.
Another solution is to use various structures to support the bushing bottom plate. The supports used in a conventional precious metal fiberglass bushing include three principal components: a center support, an external support system, and an internal gusset system. Each of these components addresses sag, or high temperature creep, of the precious metal alloy for different areas of the bushing. Each component individually supports a different part of the bushing and it is preferable to have all three components to achieve the maximum service life for a bushing.
A conventional bushing with each of these support components is shown in FIG. 3. The bushing 10 has a center support 70 and an external support system including external support straps 60 and external support brackets 62. The bushing also has an internal support system that includes tip plate gussets 44. The bushing shown in FIG. 3 includes a frame 11, side walls 16, 18, flanges 17, 19, and a V screen 15 through which molten glass flows. Filaments of molten glass are attenuated from nozzles 12 on bottom plate 14.
The center support 70 is a flattened, water cooled nickel or stainless steel tube that is mounted beneath the tip section and external to the bushing tip plate. An example of a center support is described in detail in U.S. Pat. No. 4,055,406 to Slonaker et al. (Slonaker), which is expressly incorporated by reference herein. The function of the center support is to provide support between the two tip plates for a double bottom plate bushing configuration. For a single bottom plate bushing, the center support is the only one of the three components that is not required to support the bushing bottom plate.
The center support extends the entire length of the tip plates and is isolated from the bushing by an intermediate ceramic insulator. The relationship of the center support to a double bottom tip plate is shown in FIG. 3.
Slonaker discloses a center support that includes a tubular body disposed lengthwise of and beneath the floor section of the bushing. Cooling fluid is circulated through the tubular member to minimize or reduce distortion or sagging of both the center support and the bottom plate of the bushing.
Turning to the external support system, the external support system shifts the mechanical support of the outer perimeter of a bushing from the castable refractory, which surrounds and insulates the bushing, to a bushing frame which is a more dimensionally rigid component. An example of the external support system is shown in FIG. 3 as well.
The external support system consists of stirrups that are attached to each lower bushing side wall, ceramic insulating wafers, and stainless steel support straps 60 that extend from the stirrups to the lower surface of the bushing frame 11. The external support system is intended to maintain the dimensional stability of the tip section along the perimeter of the bushing.
The center support and the external support system are external systems that support the bushing along the perimeters of each bottom plate. Support must be provided along the interior of the bottom plate as well.
The interior portion of the bottom plate is equipped with tip orifices for metering glass flow. Some conventional bushing are used with external cooling fin blades that are located between the nozzles to insure the required thermal environment for the formation of fibers. Due to the spatial requirements of the tips and the fin blades, it is necessary to support the tip plate span between the perimeter and the area between the two tip plates with a support system inside the bushing.
Internal support components are the third element of the tip plate support system and are known as tip plate gussets. The gussets are precious metal alloy vertical stiffeners that are welded to the inside surface of the tip plate between the rows of tips that are located on the tip plate. The gussets insure maximum tip plate resistance to sag or downward deflection between the externally supported perimeters of the tip plate. An example of the location of tip plate gussets 44 and their relationship with the center support and external support systems is shown in FIG. 3.
Carey acknowledges the problem of increased sagging and warping of the bushing bottom plate as the bushing increases in size. Carey discloses a modification of the third type of support (the internal support) by strengthening the gussets in a bushing. Gussets are made of precious metal alloy since they are attached to the tip plate and are located inside the bushing. The ability of the gusset to maintain tip plate flatness is directly related to how dimensionally rigid the shape of the gusset can be maintained during operating conditions of time and temperature.
The internal support system 80 in Carey, shown in FIGS. 2A and 2B, includes an elongate, internal hollow member 82 that extends between side walls of the bushing and above the bottom plate. Plates 90 are welded to the elongate members 82 and to the bushing bottom plate 14 below the members 82 at welding points 92. The plates 90 welded to the bushing bottom plate 14 between the rows of bushing tips 12. Rods 86 of high refractory ceramic material extend through passages 84 in the elongate members 82 to stiffen the members. Holes are drilled in the lower bushing side walls at each end of a gusset. The rods 82 extend through the holes in the bushing side walls to provide additional support to the gussets.
A drawback of the support assembly of Carey is that current bushings have tip plates that are longer and wider than previous bushings. The increases in width and length require numerous gussets to be installed in a single bushing. The large quantity of gussets requires additional manufacturing time and costs to drill holes for each of the ceramic rods. Further, the holes are detrimental to both the rigidity of the bushing and the heat pattern of the bushing. A bushing with numerous holes would be more vulnerable to service life limiting glass leaks around these holes and welds.
Another drawback of the system in Carey is that the height of the gussets may be too great, thereby limiting the design of any internal screens in the bushing body. The width of the tubes at the top of the gussets can also interfere with the gusset to tip plate welding sequence.
Some conventional bushings do not have nozzles and finshields depending from the bushing bottom plate in order to reduce the bushing size. While eliminating the nozzles and finshields and using only orifices may reduce the bottom plate size, it creates the problem of glass flooding the plate whenever filament breakage occurs.
There is a need for an internal support for a bushing bottom plate that does not compromise the operability of the bushing and that does not impair the flow of molten glass in the bushing. Also, there is a need for an improved bushing bottom plate support that can also serve as a perforated heater or homogenizer of the molten glass.